Electroluminescent display

ABSTRACT

An electroluminescent display panel controlled by AC flooding and exciting voltages of different frequencies for obtaining rapid response times.

Umted States Patent 1111 3,622,996

[72] Inventors James L. Bath [56] References Cited New s mn; UNITEDSTATES PATENTS fix c'Mdny'ekmviuebmhl 2,917,667 12/1959 Sack, Jr.313/108Bx 2| A I N 864 6 3,054,929 9/1962 Livingston 3l3/l08BX I f 19693,343,128 9/1967 Rogers... 340/166 d N 3,366,836 1/1968 Harvey 313/108Bx[4 1 9' 1 3,440,637 4/1969 Molnar etaL. 3l5/l69X [731 The America 3 492489 1/1970 Chynoweth 31s/169x represented by the Secretary of the NavyPrimary Examiner-Donald J. Yusko Attorneys-R. l. Tompkins, L. l. Shragoand R. K. Tendler 54 ELECTROLUMINESCENT DISPLAY 1 Claim, 4 Drawing Figs.52 U 8 CI 340 166 ABSTRACT: An electroluminescent display panelcontrolled 1 1 346/336, by AC flooding and exciting voltagesofdifferemfrequencies 511 1111.01. G08b 5/36 mmbmmmgrap'drespmet'mesi soFieldofSearch 340 166, 336;3l3/308 B;3l5/l69 Z2 22 4- 77 62 Z Z5 E/men/wn 1 /'c if lay/'6 X l I AC 46 620/221 sly/k6 1 lay/ a; 12 as 4c AL 221/14 20/16 PATENTEDunv 23 Ian SHEET 2 BF 2 5 w 2 3 0 HI: 1 p 7 m w a ma 6 v r/ 5 M d y r a 5 a S 0 MW C 5 i f 0 M a M A Z 3 44 m 3 w b r "a ou m w 4 Z 5) M1 CM FIGai DONALD 6. Mc/NTY/PE INVENTOR.

m B L S E M m iz flw/ ELECTROLUMINESCENT DISPLAY The invention describedherein may be manufactured and used by or for the Government of theUnited States of America for governmental purposes without the paymentof any royalties thereon or therefor.

This invention relates to electroluminescent display devices and, moreparticularly, to a method for obtaining rapid response times from theindividual elements in a multielement, thin-film, electroluminescentdisplay panel.

One of the main problems with a digitally addressed, thinfilm,electroluminescent-matrixed display is the relatively long turn-on timeof the phosphor elements. This turn-on time has precluded the use ofthese thin-film panels for animated displays or for the display offast-changing parameters. The concept which, when employed, permitsrapid response times for the elements is called flooding. Flooding isthe process by which each of the elements are continuously excited to apoint immediately below illumination level. When an individual elementis to be activated, a small additional voltage is applied across thiselement. The resultant turn-on time is reduced to the order of a fewhundred microseconds. Except for some design advantages, a DC, DC; a DC,AC; or an AC, AC excitation-flooding voltage may be used to decrease theturn-on times of the thin-film elements. In order to minimize elementdeterioration and flooding power requirements, AC excitation andflooding voltages having different frequencies are used in the preferredembodiment.

lt is therefore an object of this invention to provide a method andapparatus for increasing the response times of electroluminescentdevices by providing them with a flooding voltage at a point just belowtheir excitation levels.

It is a further object of this invention to provide thin-filmelectroluminescent devices with an AC source of excitation voltage andan AC source of flooding voltage.

Other objects, advantages and novel features of the invention willbecome apparent from the following detailed description of the inventionwhen considered in conjunction with the accompanying drawings.

FIG. 1 is a representation of the electroluminescent display panelshowing the application of flooding and excitation voltages thereto;

FIG. 2 is a drawing indicating the construction of a typical thin-filmelectroluminescent display panel;

FIG. 3 is a schematic diagram of one system for applying flooding andexcitation voltages to the individual elements in a thin-filmelectroluminescent display panel; and

FIG. 4 is a circuit diagram of the AC switch shown in FIG. 3.

Referring to FIG. 1, a typical electroluminescent display panel 1 with asymbolic digital display is shown. At the edges of this display areshown row conductors 2 and column conductors 3. These conductors passthrough the panel from one side to the other so that connections to theconductor may be made at either end. The aforementioned floodingvoltage, shown diagrammatically at 4, is applied at 5 and 6 to all ofthe column conductors and all of the row conductors, respectively, bythe system shown in FIG. 3. This effectively applies the floodingvoltage across each individual electroluminescent element by creating apotential across the points that each of the row conductors overlieseach of the column conductors. In the panel and as will be describedsubsequently, the row conductors are separated from the columnconductors by two insulating layers and a layer of electroluminescentmaterial sandwiched therebetween. When the flooding voltage is appliedin the manner described, a potential is created across theelectroluminescent material in much the same way as a potential isdeveloped across the plates of a conventional capacitor. The floodingvoltage is set such that the potential thus created is only slightlyless than that necessary to cause the electroluminescent material tofluoresce. The excitation voltage, shown at 7, is applied across thoserow and column conductors selected by logic 8 to activate theelectroluminescent material sandwiched between the points ofintersection of the selected row and column conductors. The potential atthese points of intersection or overlapping is thus raised above thatnecessary to cause fluorescence. The materials sandwiched betweenoverlapping conductors along with the overlapping portions of theconductors are referred to hereinafter as thinfilm electroluminescent(TFEL) elements. A typical TFEL display panel is fabricated as shown inFIG. 2.

Vacuum-deposited, electroluminescent thin-films have been employed toprovide a uniform brightness over large area display surfaces used forvisual readout. One such display panel currently available is providedwith the above crossed electrode or conductor configuration. This is a258X258 array of 66,564 individual elements in a 7-inch square activearea to provide a linear resolution of 36 elements per inch. Its activeelement is a phosphor film which, because of its lack of granularity,provides high resolution. The intersection area of a pair of crossedelectrodes or conductors accurately defines and locates the area of aspecific light-emitting element. There can be no displacement orpositional flicker of this solid-state ele ment since the position ofthe intersection does not change. The topmost electrode or conductor istransparent to enable the luminescence to be visible. Wide angle viewingis inherent in this type of display since all elements are in one planenear the top or front surface of the panel.

FIG. 2 shows the laminar construction of the panel in cross section. Thepanel is constructed on a glass substrate 10 which forms the frontsurface of the display. On top of the substrate are deposited a seriesof transparent conductors in parallel rows. One such conductor is shownin cross section at 11. In one embodiment, these conductors were tinoxide layers etched into the substrate. A layer of dielectric orinsulating material 12 is then deposited over the transparent conductorsand onto the substrate. This dielectric film is also transparent toallow the luminescence to be seen. Over the dielectric film is depositeda film of electroluminescent material 13 which in one embodiment is aphosphor. Another insulating layer or dielectric film 14 is thendeposited over the phosphor. This film has an index of refraction equalto that of the electroluminescent film. and is nearly opaque to absorbany ambient light which may enter through the substrate. If all of thislight is absorbed, the contrast between the light from the display andthe background will be at a maximum.

On top of this last insulating layer are deposited a series of metalelectrodes 15 which are layed out so that they are in parallel rowsperpendicular to the parallel rows of transparent conductors. Whenexcitation voltage 7 is applied across the phosphor, it luminesces orfluoresces as shown at 16, emitting light 17 through the substratetowards the viewer.

It will be appreciated that with the appropriate insulating materialsand phosphors crosstalk between the elements can be reduced. Crosstalkoccurs when more than one element fluoresces when only one element isactivated. A portion of the circuitry shown in FIG. 3 not only allowsfor rapid tum-on of each element but also further reduces crosstalk.

While the flooding and excitation voltages may both be DC, AC drivingand flooding voltages have the advantage of slower element deteriorationand less power drain over DC-DC or DC-AC combinations. In theabove-mentioned panel, AC voltages of 740 volts peak to peak have beenused to drive the display depending on the frequencies of the voltagesemployed.

It is a finding of this invention that flooding the panel just describeddecreases the time necessary to turn on an element to a few millisecondsfrom turn-on times on the order of seconds. A further finding is thatwith AC flooding an excitation voltage of substantially differentfrequencies not only is turn-on time greatly decreased over otherflooding excitation arrangements but current drain is diminished andelement deterioration is greatly reduced. This result was unexpectedbecause previous tests showed that as flooding voltage was increased,turn-on time also increased. However, as the flooding voltage wasincreased past a certain point, turn-on time decreased. If the floodingvoltage is set so that each element is dimly lit, the turn-on time isfaster than the response time of most photomultipliers. Even with theelements dimly lit, contrast between the activated and inactivatedelements is good. There is, however, a trade-off between fast tum-ontime and light output from the panel. As the turn-on time is made toincrease, light output decreases. With AC-AC floodingexcitation systemsin which two frequencies of AC voltage are used, this trade-off is lesscritical. in the embodiment shown in FIG. 3, a 400 Hz. flooding voltagewas used with an excitation voltage. This excitation voltage has acarrier frequency of 20 kHz. with a pulse repetition rate of 40 Hz.Flooding voltage was 480 volts peak to peak with an excitation voltageof approximately 740 volts peak to peak in the above embodiment.

FIG. 3 shows a portion of the circuit used to drive the TFEL display. Inthis embodiment, the flooding voltage is generated at supply 20 with afrequency of 400 Hz. The voltage amplitude is regulated by a variac 21to a point at which the elements shown diagrammatically at 22 are dimlylit. This voltage is coupled across each element through transformer 23and resistors 24, 25, 26 and 27, which are crosstalk suppressionresistors. Capacitors 30 and 31 are provided for additional crosstalksuppression. Without this portion of the circuit, there is no path fromsuppression resistor to ground for the excitation voltage. This is dueto the inductance of transformer 23. To complete a low-impedance pathfor the excitation voltage, capacitors 30 and 31 are added across eachside of the flooding supply transformer to center tap. The value ofthese capacitors is chosen so that impedance to the 400 Hz. floodingsupply is high compared to the impedance to the excitation voltagederived from oscillator 32 which drives power amplifier 33 at 20 kHz.with a pulse repetition rate modulation of 40 Hz. This excitationvoltage is coupled across elements 22 through transformer 34 and ACswitches 35, 36, 37 and 38. It will be appreciated that switches 36 and37 must be activated to turn on the shaded element 22. A switch capableof switching the AC excitation voltage to this element is shown in FIG.4 and discussed hereinafter. it was discovered that the slippage inphase relationship of the 400 Hz. flooding voltage and the 40 Hz. pulserepetition rate resulted in modulation of the light output of theelements. The elements would become bright or dim as the frequencychanged phases with respect to each other. A millihenry coil 39 is shownshunted across the column driver load to minimize the amplitude of thismodulation. Also, by adjustment of the frequency to obtain a maximumslippage rate of one-half the repetition rate, the flicker of the lightmodulation can be made unnoticeable.

FIG. 4 is a schematic diagram of an AC switch capable of connecting oneside of the excitation voltage to a particular electroluminescentelement. It is composed of three transistors 46, 47 and 48 and fivediodes 49, 50, 51, 52 and 53. Transistors 46 and 47 are cascaded to forma bias control for transistor 48. Diode 49 is connected in parallel withthe base to emitter of transistor 47 so as to ensure that any spuriousnoise or transient voltages that might tend to reverse bias and destroythe transistor 47 emitter to base junction will be conducted by diode 49and thereby limit reverse emitter to base voltage to approximately 0.6volts. When transistor 48 is biased into conduction, the first halfcycle of the excitation voltage flows from common driver 54 throughdiode 51, transistor 48 and diode 53 to load output 55, having beenblocked at diodes 52 and 50. The second half cycle flows from load input55 through diode 52, transistor 48 and diode 50 when transistor 48 isbiased into conduction. The second half cycle is likewise blocked atdiodes 51 and 53. This diode arrangement serves to shunt the two halfcycles of the excitation voltage through transistor 48 and, as such,completes the AC switching circuit.

What is claimed is:

l. A thin film, electroluminescent display apparatus having a fasttum-on time, a relatively low-current drain and a low deterioration ofthe light emitting material utilized, comprismg a sheet ofphotoluminescent material;

an electrical insulating layer contacting each side of said sheetrows oftransparent electrical conductors disposed on one insulating layer andcolumns of electrical conductors disposed on the other insulating layerto define a rectilinear array of light emitting areas;

means for continuously applying across said rows and columns ofelectrical conductors a unifonn AC flooding voltage of an amplitude justbelow that necessary to excite said photoluminescent material intofluorescence;

means for applying acrosspreselected conductors of said rows and saidcolumns an AC excitation voltage of a predetermined pulse repetitionrate and of a frequency that is different from said AC flooding voltageto produce an optical display; and

means for minimizing the light flicker due to the changing phaserelationship between said AC flooding voltage and said AC excitationvoltage.

1. A thin film, electroluminescent display apparatus having a fastturn-on time, a relatively low-current drain and a low deterioration ofthe light emitting material utilized, comprising a sheet ofphotoluminescent material; an electrical insulating layer contactingeach side of said sheet; rows of transparent electrical conductorsdisposed on one insulating layer and columns of electrical conductorsdisposed on the other insulating layer to define a rectilinear array oflight emitting areas; means for continuously applying across said rowsand columns of electrical conductors a uniform AC flooding voltage of anamplitude just below that necessary to excite said photoluminescentmaterial into fluorescence; means for applying across preselectedconductors of said rows and said columns an AC excitation voltage of apredetermined pulse repetition rate and of a frequency that is differentfrom said AC flooding voltage to produce an optical display; and meansfor minimizing the light flicker due to the changing phase relationshipbetween said AC flooding voltage and said AC excitation voltage.